Holding material for catalytic converter and method for producing the same

ABSTRACT

A holding material for a catalytic converter is interposed in a gap between a catalyst carrier and a metal casing receiving the catalyst carrier. The holding material includes a mat including alumina fiber and mullite fiber, wherein the alumina fiber and the mullite fiber are unitarily collected to form the holding material having a predetermined thickness.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a holding material for acatalytic converter, for example, for purging exhaust gas emitted froman automobile, and a method for producing the holding material for thecatalytic converter.

[0003] 2. Description of the Related Art

[0004] As known commonly, a catalytic converter for purging exhaust gasis mounted in a vehicle such as an automobile in order to removedetrimental components such as carbon monoxide, hydrocarbon, andnitrogen oxides from exhaust gas emitted from an engine of the vehicle.Generally, as shown in FIG. 2 which is a sectional view, the catalyticconverter has a catalyst carrier 1 shaped like a cylinder, a metalcasing 2 for receiving the catalyst carrier 1, and a holding material 3interposed in a gap between the catalyst carrier 1 and the casing 2while mounted on the catalyst carrier 1.

[0005] Generally, the catalyst carrier 1 has a cylindrical honey-combmolded material, for example, made of cordierite, and a precious metalcatalyst carried by the molded material. It is therefore necessary thatthe holding material 3 interposed in a gap between the catalyst carrier1 and the casing 2 has a function for holding the catalyst carrier 1safely to prevent the catalyst carrier 1 from being damaged by collisionwith the casing 2 due to vibration or the like during the running of theautomobile, and a function for sealing the catalyst carrier 1 to preventnon-purged exhaust gas from leaking out through the gap between thecatalyst carrier 1 and the casing 2. Therefore, the holding materialheretofore mainly used is a holding material formed by collection ofalumina fiber, mullite fiber or other ceramic fiber into a mat shapehaving a predetermined thickness or a holding material molded into acylindrical shape from the mat-like holding material.

[0006] Recently, with the movement toward more rigid regulation ofexhaust gas, there is a tendency for exhaust gas to be heated at a hightemperature in order to increase efficiency of removing detrimentalcomponents from the exhaust gas. The ceramic fiber mat however has aproblem that the ceramic fiber mat cannot be used at a temperature ofnot lower than 600° C. This is because when the ceramic fiber mat isheated while compressed, the ceramic fiber mat gets into a so-calledheat set state in which the thickness of the ceramic fiber mat cannot berestored to its original thickness even if the ceramic fiber mat isreleased from the pressure. In addition, when an inorganic expandingmaterial such as vermiculite is added to the ceramic fiber mat to form aso-called thermal expansion mat, heat resistance can be improved but thetemperature is still limited to 700-800° C.

[0007] The alumina fiber mat has an advantage in that the alumina fibermat has heat resistance to endure a high temperature of about 1,000° C.and is low in decreasing rate of elastic recovery with the elapsed time.On the other hand, the alumina fiber mat has a disadvantage in thatsurface pressure is slightly reduced in comparison with other fiber matsin equal density. Accordingly, the cost increases because it isnecessary to increase the density in order to increase the surfacepressure.

[0008] The mullite fiber mat has an advantage in that high surfacepressure can be obtained in comparison with other inorganic fiber matsin equal density. On the other hand, the mullite fiber mat however has adisadvantage in that the mullite fiber mat deteriorates with the elapsedtime because the decreasing rate of elastic recovery is high when heatstress is repeatedly applied on the mullite fiber mat.

[0009] As described above, a holding material which has high surfacepressure so that the catalyst carrier 1 can be held well, which has highsealing performance and which little deteriorates with the elapsed timehas not been obtained yet in the related art. The present situation isthat no measure can be taken in the related art against rising of thetemperature in use of the catalytic converter.

SUMMARY OF THE INVENTION

[0010] Accordingly, an object of the present invention is to provide acatalytic converter holding material in which more excellent catalystcarrier holding performance and exhaust gas sealing performance can bekept for a long term even at a high temperature.

[0011] The present inventors have made eager examination to achieve theforegoing object. As a result, it has been found that the foregoingobject can be achieved when a holding material for a catalyticconverter, which is interposed in a gap between a catalyst carrier and ametal casing receiving the catalyst carrier, comprising:

[0012] a mat including alumina fiber and mullite fiber,

[0013] wherein the alumina fiber and the mullite fiber are unitarilycollected to form the holding material.

[0014] For example, it has been found that the foregoing object can beachieved when a holding material constituted by a laminate of an aluminafiber mat and a mullite fiber mat is used in the condition that thealumina fiber mat is disposed on a side to come into contact with acatalyst carrier so that characteristics of the two mats can be combinedwith each other synergically.

[0015] That is, in order to achieve the foregoing object, the inventionprovides a holding material for a catalytic converter having a catalystcarrier shaped like a cylinder, a casing for receiving the catalystcarrier, and a holding material interposed in a gap between the catalystcarrier and the casing while mounted on the catalyst carrier, theholding material being constituted by a laminate of an alumina fiber matand a mullite fiber mat, the alumina fiber mat being constituted byalumina fiber collected into a predetermined thickness, the mullitefiber mat being constituted by mullite fiber collected into apredetermined thickness, the alumina fiber mat being disposed on a sidewhere the alumina fiber mat comes into contact with the catalystcarrier.

[0016] In the holding material according to the invention, because thealuminum fiber mat excellent in heat resistance is disposed on ahigh-temperature side to come into contact with the catalyst carrier,the deterioration of the mullite fiber mat with the elapsed time can besuppressed so that high surface pressure due to the mullite fiber matcan be kept for a long term.

[0017] Further, it has been found that the foregoing object can beachieved when a holding material is molded out of a mixture of aluminafiber and mullite fiber into a mat shape or a shape according to thecontour shape of a catalyst carrier so that characteristics of the twokinds of fiber are combined with each other synergically.

[0018] That is, in order to achieve the foregoing object, the inventionprovides a catalytic converter holding material used in a catalyticconverter having a catalyst carrier, a casing for receiving the catalystcarrier, and a holding material interposed in a gap between the catalystcarrier and the casing while mounted on the catalyst carrier, theholding material being constituted by a mixture of alumina fiber andmullite fiber collected into a mat shape having a predeterminedthickness.

[0019] The invention also provides a method of producing a holdingmaterial for a catalytic converter having a catalyst carrier, a casingfor receiving the catalyst carrier, and a holding material interposed ina gap between the catalyst carrier and the casing while mounted on thecatalyst carrier, the method including the steps of: forming a moldinghaving a flat shape or a shape corresponding to a contour shape of thecatalyst carrier by sucking and dehydrating an aqueous slurry containingalumina fiber and mullite fiber; and drying the molding.

[0020] In the holding material according to the invention, a mixture ofalumina fiber excellent in heat resistance and mullite fiber capable ofbeing expected to have high surface pressure is used so that surfacepressure sufficient to hold the catalyst carrier can be kept for a longterm.

[0021] The above-mentioned “mat” and “mixture of fiber collected into amat shape” according to the present invention may be used in a shape ofcylinder. Further, the word “mat” according to the present invention maybe used to refer to a holding material after shaped like a cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a sectional view schematically showing a catalyticconverter provided with a holding material according to the invention.

[0023]FIG. 2 is a sectional view schematically showing a catalyticconverter provided with a holding material in the related art andaccording to the invention.

[0024]FIG. 3 is a graph showing measured results of the relation betweendensity and surface pressure in the case is where the holding materialis produced while the mixture ratio of alumina fiber to mullite fiber ischanged.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] A holding material according to the present invention will bedescribed below in detail.

[0026] First Embodiment

[0027]FIG. 1 is a sectional view, like FIG. 2, schematically showing acatalytic converter provided with a holding material according to afirst embodiment of the invention. Like the related-art catalyticconverter, the catalytic converter as shown in FIG. 1 has a catalystcarrier 1 shaped like a cylinder, a metal casing 2 for receiving thecatalyst carrier 1, and a holding material 3 interposed between a gapbetween the catalyst carrier 1 and the casing 2 while mounted on thecatalyst carrier 1. The holding material 3 according to the invention isconstituted by a laminate of an alumina fiber mat 3A and a mullite fibermat 3B.

[0028] The laminate of the alumina fiber mat 3A and the mullite fibermat 3B can be shaped like a flat sheet. To hold the catalyst carrier 1,the laminate needs to be wound on the catalyst carrier 1 so that thealumina fiber mat 3A comes into contact with the catalyst carrier 1. Thelaminate may be shaped like a cylinder so that the alumina fiber mat 3Ais located on the inner side while the mullite fiber mat 3B is locatedon the outer side. The holding material 3 shaped like a cylinder can bedirectly mounted on the catalyst carrier 1 to facilitate production ofthe catalytic converter.

[0029] Mats suitably selected from known mats heretofore used in aholding material can be used as the alumina fiber mat 3A and the mullitefiber mat 3B.

[0030] The mat preferably used as the alumina fiber mat 3A is a mat, forexample, made of alumina fiber containing 90% by weight or more of Al₂O₃(and SiO₂ as a residual component) and having low crystallinity in termsof X-ray crystallography. Generally, the fiber size of the alumina fiberis preferably selected to be in a range of 3 μm to 7 μm, and the fiberlength of the alumina fiber is preferably selected to be in a range offrom 0.05 mm to 5 mm.

[0031] The mat preferably used as the mullite fiber mat 3B is a mat madeof mullite fiber having a mullite composition having an Al₂O₃/SiO₂weight ratio of about 72/28 to about 80/20 and having low crystallinityin terms of X-ray crystallography. Generally, the fiber size of themullite fiber is preferably selected to be in a range of 3 μm to 7 μm,and the fiber length of the mullite fiber is preferably selected to bein a range of from 0.05 mm to 5 mm.

[0032] A method for obtaining the laminate of the alumina fiber mat 3Aand the mullite fiber mat 3B is not limited but, for example, thelaminate can be obtained by the following method. That is, a slurrycontaining alunima fiber and an organic binder and a slurry containingmullite fiber and an organic binder are prepared respectively inadvance. First, the alumina fiber-containing slurry is molded by suctiondehydration and dried to obtain an alumina fiber mat 3A. Then, themullite fiber-containing slurry is applied on one surface of the aluminafiber mat 3A, molded by suction dehydration and dried to obtain alaminate of the alumina fiber mat 3A and a mullite fiber mat 3B. Thethus obtained laminate is compressed into a predetermined thickness.Thus, the holding material according to the invention is accomplished.

[0033] The holding material can be shaped like a cylinder as follows.First, a cylindrical mesh member (e.g., a cylindrical metal net) is usedso that the cylindrical mesh member is immersed in the aluminafiber-containing slurry. The mesh member taken out is directly subjectedto dehydration molding. The molded product is dried to produce acylindrical alumina fiber mat 3A. Then, the mullite fiber-containingslurry is applied on an outer surface of the cylindrical alumina fibermat 3A, molded by suction dehydration and dried. Finally, when thecylinder mesh member is removed, a cylindrical laminate of the aluminafiber mat 3A located on the inner side and a mullite fiber mat 3Blocated on the outer side is obtained. The thus obtained laminate iscompressed into a predetermined thickness. Thus, the cylindrical holdingmaterial is obtained.

[0034] In the invention, the density of the alumina fiber mat 3A, thedensity of the mullite fiber mat 3B, the thickness ratio between thealumina fiber mat 3A and the mullite fiber mat 3B and the totalthickness of the holding material 3 can be determined suitably inaccordance with the size, service temperature, etc. of the catalyticconverter to which the holding material 3 is applied. Generally, thedensity of the alumina fiber mat 3A and the density of the mullite fibermat 3B are both selected to be in a range of from 200 g/m² to 500 g/m².In the invention, this density range can be used. To have heatresistance due to the alumina fiber mat 3A and high surface pressure dueto the mutllite fiber mat 3B in good balance, it is preferable that thethickness of the alumina fiber mat 3A is selected to be in a range offrom 10% to 50% of the total thickness of the holding material 3 and thethickness of the mullite fiber mat 3B is selected to be in a range offrom 50% to 90% of the total thickness of the holding material 3, whenthe density of the alumina fiber mat 3A and the density of the mullitefiber mat 3B are both in the aforementioned density range.

EXAMPLE OF FIRST EMBODIMENT

[0035] The invention will be described below more specifically inconnection with the following Example and Comparative Examples but theinvention is not limited thereto at all.

Example 1

[0036] Into water, 97 parts by weight of alumina fiber of lowcrystallinity containing 96% by weight of Al₂O₃ (and SiO₂ as a residualcomponent) and having a fiber size of about 4 μm and a fiber length ofabout 3 mm, and 5 parts by weight of an organic binder (acrylicemulsion) were dispersed to prepare an alumina fiber-containing aqueousslurry. On the other hand, 97 parts by weight of mullite fiber of lowcrystallinity containing 80% by weight of Al₂O₃ (and SiO₂ as a residualcomponent) and having a fiber size of about 4 μm and a fiber length ofabout 3 mm, and 3 parts by weight of an organic binder (acrylicemulsion) were dispersed into water to prepare a mullitefiber-containing aqueous slurry.

[0037] First, the alumina fiber-containing aqueous slurry was molded bysuction dehydration and dried to produce an alumina fiber mat havinggrammage (areal density) of 400 g/m². Then, the mullite fiber-containingaqueous slurry was applied on one surface of the alumina fiber mat,molded by suction dehydration and dried to thereby laminate a mullitefiber mat having grammage (areal density) of 600 g/m² on the aluminafiber mat. The thus obtained laminate of the alumina fiber mat and themullite fiber mat was compressed to obtain a sheet-like holding materialhaving a thickness of 7.5 mm and grammage (areal density) of 1,000 g/m².

[0038] Incidentally, the thickness ratio of the alumina fiber mat to themullite fiber mat in the obtained sheet-like holding material was 33%(alumina fiber mat) to 67% (mullite fiber mat). The sheet-like holdingmaterial was compressed up to a thickness of 4 mm at a speed of 1 mm/minby a universal testing machine. Surface pressure in this condition was90 kPa. Room temperature elastic recovery of the sheet-like holdingmaterial as a whole was 75%.

Comparative Example 1

[0039] The alumina fiber-containing aqueous slurry used in Example 1 wasused so that the slurry was molded by suction dehydration, dried andcompressed to produce a sheet-like holding material having a thicknessof 7.5 mm. The sheet-like holding material was compressed up to athickness of 4 mm at a speed of 1 mm/min by the universal testingmachine. Surface pressure in this condition was 70 kPa. Room temperatureelastic recovery of the sheet-like holding material as a whole was 70%.

Comparative Example 2

[0040] The mullite fiber-containing aqueous slurry used in Example 1 wasused so that the slurry was molded by suction dehydration, dried andcompressed to produce a sheet-like holding material having a thicknessof 7.5 mm. The sheet-like holding material was compressed up to athickness of 4 mm at a speed of 1 mm/min by the universal testingmachine. Surface pressure in this condition was 100 kPa. Ordinarytemperature elastic recovery of the sheet-like holding material as awhole was 80%.

[0041] (Heat Resistance and Sealability Evaluation Test)

[0042] The sheet-like holding material obtained in Example 1 was woundon a cordierite catalyst carrier of a cylindrical honey-comb structurehaving an outer diameter of 100 mm and a length of 100 mm, so that thealumina fiber mat came into contact with the catalyst carrier. Then, theresulting product was mounted in a stainless steel casing having aninner diameter of 108 mm and a length of 120 mm to produce a catalyticconverter. For the sake of comparison, the holding materials obtained inComparative Examples 1 and 2 were used to produce catalytic convertersin the same manner as described above.

[0043] Each of the produced catalytic converters was connected to anexhaust pipe of a gasoline engine. Exhaust gas was passed through thecatalytic converter continuously for 24 hours. Gas emitted from thecatalytic converter during the passage of the exhaust gas was analyzedto examine whether exhaust gas leaked from the holding material or not.After the passage of the exhaust gas was completed, the sheet-likeholding material was taken out of each of the catalytic converters.Then, the elastic recovery of the sheet-like holding material wasmeasured, so that the decreasing rate of elastic recovery from theinitial elastic recovery was calculated. The holding material wasfurther compressed up to a thickness of 4 mm at a speed of 1 mm/min bythe universal testing machine so that surface pressure in this conditionwas measured. Leakage of exhaust gas from the holding material, thedecreasing rate of elastic recovery and surface pressure were as shownin Table 1. TABLE 1 Comparative Comparative Example 1 Example 1 Example2 Leakage of Absent Present Present Exhaust Gas Room Temperature 75 7080 Elastic Recovery (%) Room Temperature 60 55 48 Elastic Recovery (%)after Heating Decreasing Rate (%) 80 79 60 of Elastic Recovery SurfacePressure 90 70 100 (kPa) at Ordinary State Surface Pressure 80 63 50(kPa) after Test

[0044] It was confirmed from Table 1 that the holding materialconstituted by a laminate of an alumina fiber mat and a mullite fibermat in Example 1 was lower in the decreasing rate of elastic recoverythan the holding material constituted by only the alumina fiber mat(Comparative Example 1) or only the mullite fiber mat (ComparativeExample 2), that is, the holding material obtained in Example 1 wasexcellent in sealing performance.

[0045] Second Embodiment

[0046]FIG. 2 is a sectional view schematically showing a catalyticconverter provided with a holding material according to a secondembodiment of the invention. The configuration of the catalyticconverter is the same as that of a related-art catalytic converterexcept that the holding material 3 according to the invention isconstituted by a mixture of alumina fiber and mullite fiber.

[0047] The holding material 3 can be shaped like a flat mat. When theholding material 3 is molded into a shape according to the contour shape(e.g., cylindrical shape in FIG. 2) of a catalyst carrier 1, the holdingmaterial 3 can be directly mounted on the catalyst carrier 1 tofacilitate production of the catalytic converter.

[0048] Materials suitably selected from known materials heretofore usedin a holding material can be used as the alumina fiber and the mullitefiber. The material preferably used as the alumina fiber is fibercontaining 90% by weight or more of Al₂O₃ (and SiO₂ as a residualcomponent), having low crystallinity in terms of X-ray crystallographyand having a mean fiber size of 3 μm to 7 μm and a wet volume of 400cc/5 g to 1,000 cc/5 g. On the other hand, the material preferably usedas the mullite fiber is a mullite composition having an Al₂O₃/SiO₂weight ratio of about 72/28 to about 80/20, having low crystallinity interms of X-ray crystallography and having a mean fiber size of 3 μm to 7μm and a wet volume of 400 cc/5 g to 1,000 cc/5 g.

[0049] The wet volume is calculated by a method having the followingsteps:

[0050] (1) weighing 5 g of a dried fiber material by a weigher withaccuracy of two or more decimal places;

[0051] (2) putting the weighed fiber material into a glass beaker havinga weight of 500 g;

[0052] (3) putting about 400 cc of distilled water at a temperature of20-25° C. into the glass beaker prepared in the step (2) and dispersingthe fiber material into the distilled water (by an ultrasonic cleaner ifnecessary) while stirring carefully by a stirrer so that the fibermaterial is not cut;

[0053] (4) transferring the content of the beaker prepared in the step(3) into a 1,000 ml graduated measuring cylinder and adding distilledwater into the graduated measuring cylinder up to the scale of 1,000 cc;

[0054] (5) ten-times repeating a process of stirring the content of thegraduated measuring cylinder prepared in the step (4) by turning thegraduated measuring cylinder upside down while blocking an opening ofthe graduated measuring cylinder with the palm of a hand or the likecarefully to prevent water from leaking out;

[0055] (6) measuring the sedimentation volume of fiber by eyeobservation after placing the graduated measuring cylinder quietly underroom temperature for 30 minutes after the stop of the stirring; and

[0056] (7) applying the aforementioned procedure to three samples andtaking an average of the measured values as a measured value.

[0057] The mixture ratio of alumina fiber to mullite fiber is preferablyselected to be in a range of from 10:90 to 90:10 in order to have heatresistance and surface pressure in good balance. If the mixture ratio isout of the range, the object of the invention cannot be achieved becausecharacteristic of one of the two kinds of fiber is superior to that ofthe other.

[0058] A part of the alumina fiber or a part of the mullite fiber may bereplaced by at least one member selected from the group consisting ofsilica-alumina fiber, silica fiber, glass fiber, rock wool andbiodegradable fiber. By this replacement, unique characteristic of theadditive fiber can be added to the holding material 3 or reduction incost can be attained. A known or available material may be used as eachof these kinds of additive fiber. The amount of the additive fiber canbe selected suitably in accordance with the kind of the additive fiberif heat resistance and surface pressure to be able to be attainedaccording to the invention are not lowered.

[0059] The holding material 3 according to the invention can be obtainedas follows. For example, a slurry containing alumina fiber and mullitefiber and further containing the aforementioned additive fiber and anorganic binder is molded by suction dehydration. The molded product isdried and compressed into a predetermined thickness. The holdingmaterial 3 can be shaped like a cylinder by the following method. Acylindrical mesh member (e.g., a cylindrical metal net) is used so thatthe slurry is molded by suction dehydration and dried. Then, the holdingmaterial 3 shaped like a cylinder is obtained when the cylinder meshmember is removed.

[0060] In the invention, the density and thickness of the holdingmaterial 3 at the time of mounting of the holding material 3 can bedetermined suitably in accordance with the size, service temperature,etc. of the catalytic converter to which the holding material 3 isapplied. Generally, the density is selected to be in a range of from 0.2g/m² to 0.6 g/m². In the invention, this density range can be used.

EXAMPLE OF SECOND EMBODIMENT

[0061] The invention will be described below more specifically inconnection with the following Examples and Comparative Examples but theinvention is not limited thereto at all.

[0062] Alumina fiber of low crystallinity containing 96% by weight ofAl₂O₃ (and SiO₂ as a residual component) and having a mean fiber size ofabout 3 μm and a wet volume of 800 cc/5 g was mixed with mullite fiberof low crystallinity containing 80% by weight of Al₂O₃ (and SiO₂ as aresidual component) and having a mean fiber size of about 4 μm and a wetvolume of 800 cc/5 g while the mixture ratio (weight ratio) of aluminafiber to mullite fiber was changed variously to (1) 100% of aluminafiber, (2) 75% of alumina fiber and 25% of mullite fiber, (3) 50% ofalumina fiber and 50% of mullite fiber, (4) 25% of alumina fiber and 75%of mullite fiber, and (5) 100% of mullite fiber. With respect to 100parts by weight of each of the thus obtained mixtures, 8 parts, byweight of an organic binder (acrylic emulsion) were dispersed into waterin order to prepare an aqueous slurry. The aqueous slurry was molded bysuction dehydration, dried and shaped like a mat. The mat was compressedto obtain surface specific gravity (grammage) of 1200 g/m².

[0063] Each of the thus obtained molded products was heated at 700° C.for an hour so that the organic binder was removed. Then, the moldedproduct was cut into sample pieces of 25 mm square. Each of the samplepieces was compressed at a cross head speed of 1 mm/min by a universaltesting machine (manufactured by Shimadzu Corporation) so that load perdensity (surface pressure) in this condition was measured. FIG. 3 is agraph showing results of the measurement. When comparison is made inequal density, it is obvious that surface pressure decreases as themixture percentage of alumina fiber increases.

[0064] (Heat Resistance and Sealability Evaluation Test)

[0065] The holding materials constituted by a mixture of alumina fiber(25%) and mullite fiber (75%) was wound on a cordierite catalyst carrierof a cylindrical honey-comb structure having an outer diameter of 100 mmand a length of 100 mm and then mounted in a stainless steel casinghaving an inner diameter of 108 mm and a length of 120 mm to produce acatalytic converter. For the sake of comparison, the holding materialconstituted by only alumina fiber and the holding material constitutedby only mullite fiber were used to produce catalytic converters in thesame manner as described above.

[0066] Each of the produced catalytic converters was connected to anexhaust pipe of a gasoline engine. Exhaust gas was passed through thecatalytic converter continuously for 24 hours. Gas emitted from thecatalytic converter during the passage of the exhaust gas was analyzedto examine whether exhaust gas leaked from the holding material or not.After the passage of the exhaust gas was completed, the sheet-likeholding material was taken out of each of the catalytic converters. Theelastic recovery of the sheet-like holding material was measured, sothat the decreasing rate of elastic recovery from the initial elasticrecovery was calculated. The holding material after the completion ofthe passage of the exhaust gas was further compressed up to a thicknessof 4 mm by the universal testing machine so that surface pressure inthis condition was measured. Leakage of exhaust gas from the holdingmaterial, the decreasing rate of elastic recovery and surface pressureafter use were as shown in Table 2. TABLE 2 Decreasing Rate of SurfaceLeakage of Elastic Pressure Exhaust Gas Recovery after Use AluminaFiber + None 80% 82 kPa Mullite Fiber Alumina Fiber None 80% 63 kPaalone Mullite Fiber None 60% 50 kPa alone

[0067] It was confirmed from Table 2 that the holding materialconstituted by a mixture of alumina fiber and mullite fiber was lower inthe decreasing rate of elastic recovery than the holding materialconstituted by only alumina fiber or only mullite fiber, that is, theholding material constituted by a mixture of alumina fiber and mullitefiber was excellent in sealing performance.

[0068] As described above, in the holding material according to theinvention, both excellent catalyst carrier holding performance andexcellent exhaust gas sealing performance can be kept even under a hightemperature, so that improvement in heat resistance of the catalyticconverter and elongation of the life of the catalytic converter can beattained. In addition, the filling density of the holding material canbe reduced to produce high surface pressure, so that the holdingmaterial is excellent in cost performance.

[0069] Further, in the holding material according to the invention, bothexcellent catalyst carrier holding performance and excellent exhaust gassealing performance can be kept even under a high temperature, so thatimprovement in heat resistance of the catalytic converter and elongationof the life of the catalytic converter can be attained.

What is claimed is:
 1. A holding material for a catalytic converter,interposed in a gap between a catalyst carrier and a metal casingreceiving the catalyst carrier, comprising: a mat including aluminafiber and mullite fiber, wherein the alumina fiber and the mullite fiberare unitarily collected to form the holding material having apredetermined thickness.
 2. The holding material according to claim 1,wherein a first portion of the mat is a alumina fiber mat includingalumina fiber and collected into a first thickness, and wherein a secondportion of the mat is a mullite fiber mat including mullite fiber andcollected into a second thickness, and wherein the alumina fiber mat isdisposed on the mullite fiber mat to form a laminate and disposed on aside where the alumina fiber mat comes into contact with the catalystcarrier.
 3. The holding material according to claim 2, wherein thelaminate is shaped like a cylinder so that the alumina fiber mat islocated on an inner side while the mullite fiber mat is located on anouter side.
 4. The holding material according to claim 1, wherein themat is a mixture of alumina fiber and mullite fiber collected into ashape having the predetermined thickness.
 5. The holding materialaccording to claim 4, wherein the mixture of alumina fiber and mullitefiber is molded into a shape corresponding to a contour shape of thecatalyst carrier.
 6. The holding material according to claim 4, whereina mixture ratio of alumina fiber to mullite fiber is selected to be in arange of from 10:90 to 90:10.
 7. The holding material according to claim4, wherein at least one of the alumina fiber and the mullite fiber ispartially replaced by at least one member selected from the groupconsisting of silica-alumina fiber, silica fiber, glass fiber, rock wooland biodegradable fiber.
 8. A method of producing a holding material fora catalytic converter, interposed in a gap between a catalyst carrierand a metal casing receiving the catalyst carrier, said methodcomprising the steps of: preparing a first slurry including alunimafiber and a first organic binder; preparing a second slurry containingmullite fiber and a second organic binder; molding the first slurry bysuction dehydration; drying the first slurry to obtain an alumina fibermat after molding the first slurry; applying the second slurry on asurface of the alumina fiber mat after drying the first slurry; moldingthe second slurry by suction dehydration to obtain a laminate of thealumina fiber mat and a mullite fiber mat; drying the laminate.
 9. Amethod of producing a holding material for a catalytic converter,interposed in a gap between a catalyst carrier and a metal casingreceiving the catalyst carrier, said method comprising the steps of:forming a molding having one of a flat shape and a shape correspondingto a contour shape of the catalyst carrier by sucking and dehydrating anaqueous slurry including alumina fiber and mullite fiber; and drying themolding.